1. Field of the Invention
This invention relates to a cathode for an electron tube and more particularly to improvement in electron emission characteristics of the cathode.
2. Description of the Prior Art
It is now still desired to make the electron beam diameter further smaller for improvement of the resolution in a cathode-ray tube for a high resolution display, a projection picture tube for a large screen, or the like. It is also desired to increase the emission current of a cathode in such an electron tube in order to improve brightness of the image particularly in a recent large-scaled tube. Therefore, there is a high demand for a cathode which can be used at a high current density, for example, in a recent high-graded cathode-ray tube or an image pickup tube for the TV system.
Referring to FIG. 1, there is illustrated the structure of a cathode in a sectional view. Engaged with a sleeve 1 is a base 2 to which a layer 3 of an electron-emissive substance is applied. The base 2 is made of Ni containing a small amount of a reducing agent such as Si or Mg. A heater 4 for heating the electron-emissive layer 3 is provided inside the sleeve 1.
A conventional electron-emissive layer 3 is made from a powder of a composite alkaline earth metal carbonate which contains elements of Ba, Sr and Ca. A suspension which contains the powder and a binder is applied to the base 2 by a spray method or the like. The applied suspension is heated in a dynamic vacuum and then aged at a higher temperature.
In order to prepare the suspension which has a viscosity suitable for, e.g., a spray application and has a uniform adhesiveness to the base 2, the powder is usually mixed with the binder and a solvent in a ball mill for about 24 hours. Typically, an organic solvent such as butyl acetate or alcohol is used as the solvent, and nitrocellulose dissolved in an organic solvent such as butyl acetate may be used as the binder.
The alkaline earth metal carbonate layer applied to the base 2 is heated by the heater 4 in a dynamic vacuum thereby to convert it into a ternary composite oxide layer of (Ba, Sr, Ca)O. This conversion can be expressed by the following reaction formula (1), and the generated CO.sub.2 gas is evacuated by a vacuum pump. EQU (Ba, Sr, Ca)CO.sub.3 .fwdarw.(Ba, Sr, Ca)O+CO.sub.2 ( 1)
After the conversion, the composite oxide layer on the base 2 is aged at a higher temperature of 900.degree.-1100.degree. C. so that the ternary composite oxide of (Ba, Sr, Ca)O may be reduced to produce at least some of free Ba by a reducing element such as Si or Mg contained in the base 2 thereby to form the electron-emissive layer 3. Such a reducing element in the base 2 diffuses toward the interface between the composite oxide layer and the base 2, and then reacts with the composite oxide. For example, the reduction of BaO is expressed by the following formula (2a) or (2b). EQU 2BaO+Si=2Ba+SiO.sub.2 ( 2a) EQU BaO+Mg=Ba+MgO (2b)
When part of BaO in the composite oxide layer is reduced to free Ba, the layer becomes a semiconductor of an oxygen deficient type. Consequently, the layer 3 of the electron-emissive substance is obtained and it can be used at a current density of 0.5-0.8 A/cm.sup.2 at an operating temperature of 700.degree.-800.degree. C.
With the conventional cathode, an emission current density higher than that above described can not be obtained for the following reasons .circle.1 and .circle.2 . .circle.1 As a result of the reaction during the aging, an intermediate layer of an oxide such as SiO.sub.2 or MgO is formed between the base 2 and the electron-emissive layer 3, so that the current is limited by a high resistance of the intermediate layer. .circle.2 The reduction of the alkaline earth metal oxide is limited by the intermediate layer and thus a sufficient amount of free Ba is not produced.
As described above, the conventional cathode can not be used at a high current density. Further, there exists a problem that since the conventional electron-emissive layer 3 is of a semiconductor, the layer 3 may be destroyed thermally due to the Joule heat at a high current density.